This invention relates to dye compositions and, more particularly, to stable, aqueous dye suspensions for use in film coating tablets and the like to produce smooth, uniform, substantially non-mottled coatings.
Heretofore, it has been the practice to utilize FD&C lakes rather than water-soluble dyes as the materials of choice for coating tablets and the like. The problems attending coating tablets with water-soluble dyes has been noted in various patents and publications. For example, in an article entitled "Characterization Techniques for the Aqueous Film Coating Process by Mathur et al., Pharmaceutical Technology, October 1984, it is noted that when water-soluble dyes are used in aqueous film coating, problems with color migration or mottling upon drying are encountered.
The dynamics of coating include the spreading of a thin wet layer of a coating suspension on a tablet which is dried rapidly after spreading. The drying dynamics usually lead to uneven distribution of a water-soluble dye in the dried layer. Though the color can be uniformly distributed in the coating suspension and applied very uniformly on the tablet, a mottled appearance normally results when water-soluble dyes are used. This is believed to be due to the dye molecules migrating in the film, being transported by the aqueous medium as it moves in the film layer surface and evaporates at distinct points. The dye thus accumulates at these points producing dark patches or mottling.
The use of lakes has overcome these problems because they are insoluble pigments which are manufactured by attaching dye molecules to aluminum hydroxide particles thereby rendering them water-insoluble. These pigment particles or lakes are then dispersed in a coating suspension and applied uniformly to the surface of a tablet to produce a uniformly colored product because the pigment particles or lakes do not migrate in an aqueous medium.
Prior to 1960, it was the practice to sugar coat tablets and candies with water-soluble dyes dissolved in sugar syrup. However, since the development of lakes in 1959, most sugar coating is carried out with lakes. Film coating of tablets and the like with water-soluble dyes has never been successfully achieved due to the difficulty of obtaining uniform coating because of migration of the dyes in an aqueous medium.
It has also been known to utilize mixtures of lakes and water-soluble dyes in coating suspensions with the dyes being incorporated to reduce the cost and to obtain brighter color shades.
Because water-soluble dyes are less costly than lakes, it would be advantageous if such dyes could be used in coating suspensions in lieu of lakes while yet providing uniform, non-mottled coatings.